Oh dear Sid,this is probably iris rhizome rot,caused by a bacterium in damp conditions.If you can cut back to healthy growth you may be able to save this one.Check out any others you have too.Good luck :)

Uh oh. It's fairly sharply drained there Aster and it's been very dry here recently...the effected bit is right on the edge of the clump, which is a bit congested, if that makes a difference...?

There may be moisture trapping there then Sid.Do you think they are due for dividing soon ?
We've had such a damp winter the damage may have started long ago :(

Yeah, they do need dividing this year, but they look healthy enough - lots of flower spikes... Do you think this problem is likely to spread? I've planted some new ones a couple of feet away :-(

The new ones should be ok but I would remove the most badly affected ones.
You said it's mushy,does it smell as well ?
The bacterium enters through wounds so check for any slugs which may be damaging them.Also take care when hoeing.
It sounds as though you have them in good conditions and if the others are planted shallowly they should be fine.

Thanks Aster. I didn't notice a smell, but I didn't get my nose too close lol I will remove the affected part tomorrow after work.

Hi Sid, I have had this problem on a couple of irises too. I have lifted and separated the clump, but I broke off and destroyed the main infected rhizome and kept the babies and potted them on. If they are okay in spring I will plant them back. Also I have marked the pots with the babies in so I dont pass on infected plants. There is a sour to putrid smell from the infection depending on how far gone.